Diffusing acoustical crosstalk

ABSTRACT

When two loudspeakers play the same signal, a “phantom center” image is produced between the speakers. However, this image differs from one produced by a real center speaker. In particular, acoustical crosstalk produces a comb-filtering effect, with cancellations that may be in the frequency range needed for the intelligibility of speech. Methods for using phase decorrelation to fill in these gaps and produce a flatter magnitude response are described, reducing coloration and potentially enhancing dialogue clarity. These methods also improve headphone compatibility and reduce the tendency of the phantom image to move toward the nearest speaker.

BACKGROUND

1. Field of the Invention

The invention relates to audio systems. More specifically, the inventiondescribes a method and apparatus for using phase decorrelation tominimize the effects of acoustical crosstalk.

2. Related Art

There are a number of acoustical phenomena that are rarely noticedconsciously by the average listener in a typical environment butnevertheless detract from optimal audio quality. One is acousticalcrosstalk, which occurs when two loudspeakers play the same signal,creating a phantom center image. It is well known that acousticalcrosstalk produces comb filtering with deep spectral notches, resultingin undesirable coloration and a loss of spectral information.

When two loudspeakers play the same signal, the resulting phantom centerimage differs from one produced by a real center speaker. In particularand as noted, acoustical crosstalk produces a comb-filtering effect,with cancellations that are typically in the frequency range needed forthe intelligibility of speech. In addition, the phantom image is not asstable as that of a real center speaker, because it tends to follow thelistener toward the nearest speaker due to the precedence effect. Thereare additional problems relating to mono-compatibility andspeaker/headphone compatibility.

One solution to problems of phantom center images is simply to add areal center speaker. This approach had the advantage of providing astable center image. However, for reasons of cost and space, manyconsumer audio and television systems do not include a center speaker.Therefore, an approach that works over two speakers is desired.

Another solution to the problem of acoustic crosstalk is to cancel itbefore it happens, using various crosstalk cancellation techniques.However, at mid and high frequencies, this is effective only within arelatively small “sweet spot,” which limits the usefulness of thistechnique for typical television viewing and other situations involvingmultiple listeners in arbitrary positions.

Another way to address the non-flat magnitude response caused byacoustical crosstalk is to apply inverse filters to the left and rightsignals. However, the frequencies of the comb filter notches varygreatly depending on the relative positions of the speakers andlistener. For example, the cancellation frequencies increase as theangle subtended by the speakers becomes narrower, such as when thelistener moves further back. In addition, as the listener moves to theside and is no longer equidistant from the speakers, the notches movecloser together and become different for each ear. Without a goodestimate of the relative positions, it would be impossible to accuratelyequalize the effects of the crosstalk.

SUMMARY OF THE INVENTION

In one embodiment, a method of diffusing a signal using phasedecorrelation at high frequencies for a mono input signal is described.A mono input signal is received and separated into a high-frequencysignal and a low-frequency signal. The high-frequency signal isprocessed using a diffusion means, such as an allpass filter, creating ahigh-frequency left channel signal. A second diffusion means, such as asecond non-identical allpass filter is used to process thehigh-frequency signal, creating a high-frequency right channel signal.As a result of these processes, a frequency-dependent delay is createdbetween the high-frequency left channel signal and the right channelsignal. The low-frequency signal is processed to create a delayedlow-frequency signal. The delayed low-frequency signal is combined withthe high-frequency left channel signal. The low-frequency signal is alsocombined with the high-frequency right channel signal. Thesecombinations produce a stereo response with phase diffusion at highfrequencies.

In another embodiment, a method of diffusing a signal using phasedecorrelation at high frequencies for a stereo input signal isdescribed. A left input signal is separated into a left high-frequencysignal and a left low-frequency signal. Similarly, a right input signalis separated into a right high-frequency signal and a rightlow-frequency signal. An allpass filter, or other diffusion means, isapplied to the left high-frequency signal, thereby creating an allpassedleft high-frequency signal. Another diffusion means, such as a secondnon-identical allpass filter is applied to a right high-frequencysignal, thereby creating an allpassed right high-frequency signal. Adelayed left low-frequency signal and a delayed right low-frequencysignal are created. The delayed left low-frequency signal is combinedwith the allpassed left high-frequency signal. The delayed rightlow-frequency signal is combined with the allpassed right high-frequencysignal. These combinations produce a stereo response with phasediffusion at high frequencies.

Another embodiment is a system for diffusing a mono input signal usingphase decorrelation at high frequencies. The system may consist of ahigh pass filter that accepts a mono input signal and outputs ahigh-frequency signal. Similarly, a low pass filter outputs alow-frequency signal from the mono input signal. Two allpass filters orother diffusion means create a high-frequency left channel signal and ahigh-frequency right channel signal. The allpass filters are notidentical. Other types of diffusion means may be used, such as reverb. Adelay component creates a delayed low-frequency signal that is inputinto two adders; one combines the low-frequency signal with thehigh-frequency left channel signal and another combines thelow-frequency signal with the high-frequency right channel signal.

Another embodiment is a system for diffusing a stereo input signalhaving a left input and a right input using phase decorrelation at highfrequencies. The system has a pair of filters consisting of a low passfilter and a high pass filter for processing the left input of thestereo signal. Another pair, also consisting of a low pass filter and ahigh pass filter, processes the right input of the stereo signal. Thesystem also has two allpass filters, one for creating a high-frequencyleft channel signal and another for creating a high-frequency rightchannel signal. A delay component creates a delayed low-frequency leftchannel signal and another delay component creates a delayedlow-frequency right channel signal. The high-frequency left channelsignal and the delayed low-frequency left channel signal are combinedusing an adder. Another adder is used to combine the delayedlow-frequency right channel signal and the high-frequency right channelsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

References are made to the accompanying drawings, which form a part ofthe description and in which are shown, by way of illustration,particular embodiments:

FIG. 1 is a simplified top-down view of an asymmetrical listeningenvironment;

FIG. 2A is a block diagram of a system demonstrating acousticalcrosstalk;

FIG. 2B is a graph showing a typical magnitude response resulting fromthe crosstalk depicted in FIG. 2A;

FIG. 3 shows a system for phase diffusion using a modified “Schroederquasi-stereo” circuit with arbitrary gain, g.

FIGS. 4A and 4B are graphs of left and right impulse responses fromadders shown in FIG. 3;

FIG. 4C is a graph showing left and right phase responses as a functionof frequency;

FIG. 4D is a graph showing the magnitude response of a simple delaymodel of acoustic crosstalk, at one ear, with speakers at ±30 degrees,with and without the crosstalk diffusion;

FIG. 5 is a block diagram of a system of complementary crossover filtersand allpass filters capable of limiting phase diffusion to higherfrequencies for a mono input signal in accordance with one embodiment;

FIG. 6 is a flow diagram of a process of phase diffusion of highfrequencies of a mono input signal in accordance with one embodiment;

FIG. 7 is a graph showing phase responses of left and right outputs ofadders shown in FIG. 5;

FIG. 8 is a diagram of a system for high-frequency phase diffusion for astereo input signal using complementary crossover filters and allpassfilters in accordance with one embodiment;

FIG. 9 is a flow diagram of a process of phase diffusion of highfrequencies of a stereo input signal in accordance with one embodiment;and

FIG. 10 is an efficient implementation of a magnitude-complementaryfilter pair.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to a particular embodiment of theinvention, an example of which is illustrated in the accompanyingdrawings. While the invention is described in conjunction with theparticular embodiment, it will be understood that it is not intended tolimit the invention to the described embodiment. To the contrary, it isintended to cover alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims.

Methods and systems for creating a flatter magnitude response as anapproach to alleviating phantom center image issued from acousticalcrosstalk are described in the various figures. Acoustical crosstalkoccurs when the same signal from a pair of speakers reaches the ear atslightly different times. While the resulting phase differencesfacilitate the stereo illusion at low frequencies, they also create acomb-filtering effect having a series of magnitude notches across thefrequency spectrum. This coloration not only implies that the phantomcenter image will always sound somewhat different from a real centerspeaker, but it may also reduce the intelligibility of speech.

FIG. 1 is a simplified top-down view of an asymmetrical listeningenvironment. Two loudspeakers 102 and 104 are shown at the upper leftand right of a space 106. A listener's head is represented by circle108, and speaker-to-ear paths are shown by diagonal lines 110, 112, 114and 116, labeled as transfer functions H_(LL), H_(LR), H_(RR) andH_(RL), described below. As can be seen, H_(LL) line 110 and H_(LR) 112are shorter than H_(RR) line 114 and H_(RL) line 116. Even in theunlikely case that the center of the listener's head (circle 108) islocated exactly along the plane of symmetry between speakers 102 and104, neither of the listener's ears will be located on the plane ofsymmetry, assuming the listener is facing forward. At each ear, the dualmono signal will be received from the two sources (speakers 102 and 104)with different time delays.

Acoustical crosstalk can be modeled or demonstrated by a system as shownin FIG. 2A which shows a simplified phantom mono acoustical crosstalkmodel. In this model, transfer functions H_(LL)(z) 204 and H_(RR)(z) 208represent ipsilateral acoustical paths (or paths that are the same sideof the listener) from left speaker to left ear H_(LL) 110 and from rightspeaker to right ear H_(RR) 114, respectively, and H_(LR)(z) 206 andH_(RL)(z) 210 represent contralateral acoustical paths (or paths thatare on opposite sides of the listener) from left speaker to right earH_(LR) 112 and from right speaker to left ear H_(RL) 116, respectively.A mono input signal 202 is transmitted to a right speaker and leftspeaker (not physically shown in FIG. 2A) with a gain of 0.5 in eachchannel. The left speaker signal is input to two acoustical transferfunctions: H_(LL)(z) shown as box 204 and H_(LR)(z) shown as box 206.The right speaker signal is input to two acoustical transfer functions:H_(RR)(z) shown as box 208 and H_(RL)(z) shown as box 210. The outputsof acoustical transfer functions 204 and 210 are combined or added byadder 212 producing a left channel signal 213 and heard by thelistener's left ear. The outputs of acoustical transfer functions 206and 208 are combined by adder 214 producing a right channel signal 215and heard by the listener's right ear. In this model, the transferfunctions from a mono input to the two ears are given by:

H _(L)(z)=0.5┌H _(LL)(z)+H _(RL)(z)┐, and

H _(R)(z)=0.5[H _(LR)(z)+H _(RR)(z)].

Putting aside details such as head-shadowing, which creates a region ofreduced amplitude of a sound due to obstructions from a listener's head,and focusing only on the phase cancellations, the functions can bemodeled as:

H _(L)(z)=0.5[z ^(−LL) +z ^(−RL)] and

H _(R)(z)=0.5[z ^(−LR) +z ^(−RR)]

where LL and RR are the ipsilateral delays and LR and RL are thecontralateral delays, measured in samples.

A typical magnitude response resulting from the crosstalk depicted inFIGS. 1 and 2A is shown by the dotted line 216 in FIG. 2B The solid line218 in FIG. 2B represents simulated comb filtering response for aspherical head model.

Whenever acoustical delays from the left and right speakers to a singlepoint (such as one ear) are unequal, there will be a series offrequencies at which the signals are 180° out of phase. Even if theright amount of electrical delay is added to equalize the acousticaldelays from the left and right speakers to the left ear, the totaldelays will then be unequal at the right ear.

However, for intelligibility of speech consonants, it is not necessaryto have a flat magnitude response at every frequency, due to the ear's“auditory filters.” The ear assigns the same perceived loudness tonarrow-band noise sources, regardless of the noise bandwidth, so long asthat bandwidth is less than a critical bandwidth. Thus, even if thereare cancellations within a given critical band, what is important is thetotal noise power within that band. This eliminates the need to have aflat magnitude response at all frequencies and, consequently, simplifiesthe problem considerably. In one embodiment, decorrelation of the phasedifferences between channels, within each critical band, as describedbelow, effectively randomizes the cancellations and reduces theirperceived effect. The term decorrelation may have different meanings invarious contexts. Generally, it may refer to any process for reducingcross-correlation within a set of signals while preserving other aspectsof the signals. In the current context, decorrelation transforms anaudio signal, or a pair of related audio signals, into multiple outputsignals having waveforms that look different from each other but soundthe same.

There are a number of methods of generating diffused, decorrelatedsignals that are known in the field of acoustical engineering, includingFeedback Delay Network (FDN) reverbs and convolution with time-limitedwhite noise or “velvet noise.” In one embodiment, phases between twospeakers are decorrelated, while allowing the output of each speaker tobe approximately allpass, that is, having unity gain at all frequencies.An allpass filter is one which generally allows all frequencies through.The amplitude response of an allpass filter is one at each frequencywhile the phase response can be arbitrary. This is beneficial in caseswhere a listener is seated closer to one speaker than to another. FIG. 3shows a system for phase diffusion using a modified “Schroederquasi-stereo” circuit with arbitrary gain, g. As is known in the art, aSchroeder quasi-stereo circuit was originally designed to produce apseudo-stereo effect by creating phase differences using a pair ofallpass filters. In Schroeder's original circuit, each output wasallpass only if the feedback and feedforward gains equaled ±√{squareroot over (0.5)}; the current embodiment uses a different topology toallow more flexibility in the choice of gains. A mono signal 302 isinput to two allpass filters. A left allpass filter 304 consists ofadders 306 and 308, gains 310 and 312, and N-sample delay 314. A rightallpass filter 316 consists of adders 318 and 320, gains 322 and 324,and N-sample delay 326.

Adder 306 adds mono input signal 302 to the output of feedback gain 310and sends the result to feedforward gain 312 and N-sample delay 314.N-sample delay 314 delays its input by N samples and sends the delayedsignal to feedback gain 310 and adder 308. Adder 308 adds the output ofN-sample delay 314 to the output of feedforward gain 312 and sends theresult to the left speaker.

Adder 318 adds mono input signal 302 to the output of feedback gain 322and sends the result to feedforward gain 324 and N-sample delay 326.N-sample delay 326 delays its input by N samples and sends the delayedsignal to feedback gain 322 and adder 320. Adder 320 adds the output ofN-sample delay 326 to the output of feedforward gain 324 and sends theresult to the right speaker.

Left and right allpass filters 304 and 316 are identical, except that inleft allpass filter 304, the feedback gain 310 is positive (+g) and thefeedforward gain 312 is negative, while in right allpass filter 316, thefeedback gain 322 is negative and the feedforward gain 324 is positive.Therefore, while the impulse responses of the two filters are bothallpass, the impulse responses are different due to the sign differencesbetween the gains. Therefore the phase responses are different,producing envelope delay differences as a function of frequency, wherean envelope delay generally is the propagation time delay undergone byan envelope of an amplitude modulated signal as it passes through afilter.

The system shown in FIG. 3 has allpass transfer functions A_(L)(z) andA_(R)(z), as follows:

A _(L)(z)=− g+z ^(−N), and

1−gz^(−N)

A _(R)(z)= g+z ^(−N)

1+gz^(−N)

FIGS. 4A and 4B are graphs of the left and right impulse responses fromadder 308 and adder 320, respectively, in FIG. 3. The y-axis measuresamplitude and the x-axis measures time (in samples). The impulseresponses shown in FIGS. 4A and 4B are for a gain of g=0.414. Note thatthe decays are exponential (with the exception of the first pulse), andalternate pulses are opposite in sign at the two outputs. The impulseresponses are power-complementary (since both are allpass), that is,they are energy-preserving at all frequencies, but they are not in factallpass complementary, because the phasor sum of the two outputs doesnot have constant magnitude. Therefore, they are not exactlymono-compatible. However, the system shown in FIG. 3 would normally beused for playback, not for encoding or signal transmission, so therewould be no need to mix the output back to mono.

The system of FIG. 3 results in the left and right phases beinginterleaved so that the left speaker leads at some frequencies and lagsat others. The number of alternating “bands” corresponds to a delaylength N. (If more than two speaker signals are needed, additionaldecorrelated outputs can be created by using different values of N.)

The left and right phase responses, as a function of frequency, areshown in FIG. 4C in which the y-axis measures the phase response indegrees and the x-axis represents frequency in Hz. The solid line 402represents the phase response of the left channel output from adder 308in FIG. 3, while the dash-dot line 404 represents the phase response ofthe right channel output from adder 320.

It is preferable for delay N (measured in samples) to be long enough sothat there are at least one or two alternating phase bands within eachcritical band of interest, in order to diffuse or perturb thecancellation patterns and smooth the perceived frequency response. Thealternating phase bands are spaced linearly, with a spacing of

${b = \frac{f_{s}}{2N}},$

where f_(s) is the sample rate in Hz. As is known in the art, theEquivalent Rectangular Bandwidth (ERB) provides an approximation of thebandwidth of filters used in human hearing, modeling the filters asrectangular allpass filters. The ERB of the human auditory filters isapproximated by

ERB=24.7(0.00437F+1),

where F is the center frequency in Hz. Assuming the lowest critical bandof interest is centered near the lowest comb filter notch, which may bearound 2 kHz, the smallest ERB of interest would be about 241 Hz. Inorder for the width b of our alternating phase bands to be less than theERB, we have

$N > {\frac{f_{s}}{{2 \cdot 24.7}\left( {{0.00437\; F} + 1} \right)}.}$

In this case, given a 48 kHz sampling rate, the delay N would be atleast 100 samples, or about 2 ms.

While delay N needs to be sufficiently long, as described above, it isalso preferable to avoid unnecessarily long values of N that might causeperceptible temporal smearing of impulsive sounds. Temporal smearing maybe described generally as a spreading of transient or impulsive soundsover a longer period of time. If the impulse response is viewed as atype of reverberation, the reverberation time is given by:

${T_{r} = \frac{{- 60}N}{g_{d\; B} \cdot f_{s}}},{where}$

T_(r) is the −60 dB reverberation time in seconds;

N is the length of the delay in samples;

f_(s) is the sample rate in Hz; and

g_(dB) is allpass gain g expressed in dB.

Therefore, the reverberation time is proportional to the delay time andinversely proportional to the log of the gain.

With N=100, and g=0.414, for example, the −60 dB reverberation timeT_(r) is about 16 ms. This is a short decay time compared to that ofmost rooms, so the temporal smearing is unlikely to be perceptible overspeakers with typical voice or music recordings. The values of allpassgains g and delays N can be tuned as desired to balance the variousperceptual effects.

FIG. 4D is a graph showing the magnitude response of a simple delaymodel of the acoustic crosstalk, at one ear, with speakers at ±30degrees. The dotted line 408 has deep cancellation notches, such as 410a and 410 b, caused by acoustic crosstalk alone. One of the goals is tofill in these notches or gaps, which is accomplished to a large degreeby the phase diffusion method described herein. In one embodiment, usingthe system of FIG. 3, the total magnitude response at one ear is shownby solid line 406. Note that the phase diffusion helps fill incancellation notches 410 a and 410 b in dotted line 408, caused byacoustic crosstalk. While the diffusion introduces new notches as seenin solid line 406, these are smaller and closer together, and will besmoothed by the ear's auditory filters.

A drawback of the system depicted in FIG. 3 is that the phase diffusionis applied at all frequencies, including low frequencies where phase isan important localization cue. We would prefer to diffuse the left andright phases around 2 kHz (the approximate frequency of the lowestcancellation notch 410 a in the example shown in FIG. 4D) and higher,without affecting the phase response at the lower frequencies.

Since the ear's use of phase as a localization cue (that is, a cue toascertain the direction of a sound source) is primarily limited tofrequencies below about 1 kHz, and since one of the objectives of thevarious embodiments is to diffuse the left and right phases around 2 kHzand above, a pair of complementary crossover filters can be used (asshown in FIGS. 5 and 8 below) to limit phase diffusion to frequenciesabove a selected crossover frequency.

FIG. 5 is a block diagram of a system of complementary crossover filterscapable of limiting phase diffusion to higher frequencies for a monoinput signal in accordance with one embodiment. The mono input signal isapplied to high pass filter 502 and low pass filter 504. The value ofthe low pass/high pass crossover cutoff frequency can be tuned asdesired, but will typically be 1000 Hz or higher. The high frequenciesoutput from high pass filter 502 are processed with a gain 503 of thesquare root of 0.5 (shown as 0.7) in order to normalize the reverberantsound pressure produced by the pair of allpass filters. The output ofgain 503 is applied to allpass filter 506 and allpass filter 508. Theseallpass filters are used as a means for diffusion. In other embodiments,other diffusion means, such as reverb may be used. The low frequenciesoutput from low pass filter 504 are processed with a gain 505 of thesquare root of 0.5 (shown as 0.7), again to normalize the reverberantsound pressure. The output of gain 505 is applied to delay 510, whichdelays the low-frequency path to match the average delay caused byallpass filters 506 and 508 in the high-frequency paths. The (lowfrequency) output of delay 510 is added to the (high frequency) outputof allpass filter 506 using adder 507, and the result is sent to theleft speaker. The (low frequency) output of delay 510 is also added tothe (high frequency) output of allpass filter 508 using adder 509, andthe result is sent to the right speaker. As a result, the left and rightoutputs have equal phase delays at low frequencies to preservelocalization cues, and interleaved phase delays at high frequencies todiffuse the cancellation notches.

In one embodiment, the system shown in FIG. 5 may be implemented, forexample, in a preprocessing chip or in firmware of an audio digitalsignal processor (DSP) of a television. In another example, the systemmay be implemented in a sound system amplifier.

FIG. 6 is a flow diagram of a process of phase diffusion of highfrequencies of a mono input signal in accordance with one embodiment. Atstep 602 the system receives a mono input signal. At step 604 a monoinput signal is processed by a high pass filter and a low pass filter,effectively splitting the signal into high and low frequencies. In oneembodiment, a gain of the square root of 0.5 is applied to the outputsof the high pass and low pass filters. In other embodiments, othervalues for the gain may be applied. At step 606 the phase responses ofthe high frequencies of the input signal are diffused using twonon-identical allpass filters or other diffusion means, such as reverb,one for the left channel and another for the right channel. The allpassfilters apply phase diffusion only to the high frequencies. At step 608the low frequencies are delayed so that the phase delay of thelow-frequency path matches the average phase delay of the two allpassfilters in the high-frequency paths, so the low-frequency andhigh-frequency paths are essentially synchronized. At step 610 the lowfrequencies are added with the left channel allpass filter output andwith the right channel allpass filter output, as shown in FIG. 5.Finally, the left channel signal is outputted by a left speaker and theright channel signal is outputted by a right speaker.

In FIG. 7, the two outside, interweaving curves 702 and 704 representthe phase responses of the left and right outputs of adders 507 and 509in FIG. 5, while center curve 706 is the phase response of the output ofthe lowpass filter plus N-sample delay. This delay is included in orderto compensate for the average phase response of the allpass filters andto prevent unnecessary cancellations. It is apparent from FIG. 7 thatthe phase diffusion is limited to the higher frequencies. This helpsdisrupt the phantom mono phase cancellations without adversely affectinglow frequency phase-based spatial cues.

As noted, the system in FIG. 5 is designed to convert a mono input toleft and right outputs in order to produce a flatter magnitude response,that is, a less problematic phantom center image. A system designed towork with stereo inputs is shown in FIG. 8. Here, the left and rightinputs are processed separately, with allpass filters applied to thehigh pass filtered signals.

FIG. 8 shows a system for high-frequency phase diffusion for a stereoinput signal using complementary allpass crossover filters in accordancewith one embodiment. The system shown in FIG. 8 has components similarto those in FIG. 5. A stereo input signal consists of a left inputsignal 800 and a right input signal 803. Left input signal 800 is sentto high pass filter 802 and low pass filter 804. Left channel highfrequencies passed by high pass filter 802 are sent to allpass filterA_(L) 810, and the left channel low frequencies passed by low passfilter 804 are sent to delay 814. The outputs of allpass filter A_(L)810 and delay 814 are added together in adder 811 and sent to the leftspeaker. The right input signal is sent to high pass filter 806 and lowpass filter 808. The right channel high frequencies passed by high passfilter 806 are sent to allpass filter A_(R) 812, and the right channellow frequencies passed by low pass filter 808 are sent to delay 816. Theoutputs of allpass filter A_(R) 812 and delay 816 are added together inadder 813 and sent to the right speaker. As described above, delays 814and 816 are used to synchronize the low-frequency paths with the averagedelay of the high-frequency paths. Allpass filters A_(L) and A_(R) aresimilar but different from each other; for example,

${{A_{L}(z)} = \frac{{- g} + z^{- N}}{1 - {gz}^{- N}}},{and}$${A_{R}(z)} = {\frac{g + z^{- N}}{1 + {gz}^{- N}}.}$

As a result, any high-frequency phantom center content common to theleft and right channels will be processed by A_(L) for one output and byA_(R) for the other, resulting in interweaving phase responses (phasediffusion) at high frequencies. At low frequencies, the left and rightchannels will be delayed by equal amounts, preserving low frequencyphase-based spatial cues.

FIG. 9 is a flow diagram of a process of phase diffusion of highfrequencies of a stereo input signal in accordance with one embodiment.At step 902 the system receives a left channel input signal and a rightchannel input signal. At step 903, each signal is split into high andlow frequencies by a high pass and low pass filter. At step 904 the leftand right channel high frequencies are processed separately usingnon-identical allpass filters. This creates interweaving phase delaysbetween the left and right channels at high frequencies, diffusing thesound and breaking up the phase cancellations. At step 907 the left andright channel low-frequency paths are delayed to synchronize with theaverage delay of the high-frequency paths resulting from the allpassfilters. At step 908 the high and low frequencies of each channel areadded to form left and right output signals that are phase diffused onlyabove the specified crossover frequency.

The crossover filters help minimize any increase in apparent imagewidth, for example, the width of the phantom center image, because thephases in the low-frequency range, where phase is a primary localizationcue, are not being diffused. In practice, a slight spreading orpseudo-stereo effect may still be apparent, especially when the speakerssubtend an angle of greater than ±60°, however the widening is subtle,and not unpleasant for the smaller angles typically used for televisionviewing.

For listeners to the left or right of the line of symmetry between thespeakers, the widening of the image causes the phantom center image'spull toward the nearest speaker to be somewhat less obvious. While thephantom image is still not centered exactly between the speakers, it isno longer so tightly focused toward one side.

When power-complementary crossover filters are used with the systems ofFIGS. 5 and 8, undesired fluctuations of the power response can be notedin the vicinity of the crossover frequency, due to the interaction withthe allpass filters. In a preferred embodiment, these can be minimizedusing magnitude-complementary filters, which have matching phaseresponses at all frequencies. A suitable lowpass response in oneembodiment is

$\begin{matrix}{{G(z)} = {0.5^{2}\left\lbrack {{A_{1}(z)} + {A_{2}(z)}} \right\rbrack}^{2}} \\{{= {0.25\left\lbrack {{A_{1}^{2}(z)} + {2{A_{1}(z)}{A_{2}(z)}} + {A_{2}^{2}(z)}} \right\rbrack}},}\end{matrix}$

and the corresponding highpass response is

$\begin{matrix}{{H(z)} = {- {0.5^{2}\left\lbrack {{A_{1}(z)} - {A_{2}(z)}} \right\rbrack}^{2}}} \\{{= {- {0.25\left\lbrack {{A_{1}^{2}(z)} - {2{A_{1}(z)}{A_{2}(z)}} + {A_{2}^{2}(z)}} \right\rbrack}}},}\end{matrix}$

where G(z) is the lowpass response, H(z) is the highpass response, and

A1(z) and A2(z) are stable allpass transfer functions such that

E(z)=0.5[A ₁(z)+A ₂(z)] and

F(z)=0.5[Λ₁(z)−Λ₂(z)],

where E(z) is a lowpass prototype filter, and F(z) is a correspondinghighpass filter, such that

G(z)=E ²(z), and

H(z)=−F ²(z).

A known efficient implementation of this magnitude-complementary filterpair is shown in FIG. 10. An input signal is scaled by 0.25 in gain1002, the output of which is sent to allpass filter A₂(z) 1004 andallpass filter A₁(z) 1006. The output of allpass filter A₂(z) 1004 issent to another allpass filter with the same transfer function A₂(z)1008. The output of allpass filter A₁(z) 1006 is sent to another allpassfilter with the same transfer function A₁(z) 1010, as well as to anotherallpass filter with transfer function A₂(z) 1012. The output of allpassfilters 1008 and 1010 are added in adder 1014. The output of allpassfilter 1012 is scaled by 2.0 in gain 1016. The output of adder 1014 isadded to the output of gain 1016 in adder 1018, yielding lowpass outputsignal G(z). The output of adder 1014 is subtracted from the output ofgain 1016 in adder 1020, yielding highpass output signal H(z).

Decorrelating the left and right signals simply by adding earlyreflections or reverberation might unnecessarily color the frequencyresponse or lengthen the impulse response. Furthermore, systems thatdecorrelate audio by creating magnitude differences in alternatingfrequency bands (for example, using pseudo-stereo comb filters) wouldcreate timbre problems for listeners located closer to one speaker thananother. In addition, without the crossover filters shown in FIGS. 5 and8, the resulting full-spectrum decorrelation would impose unwanted phasechanges at low frequencies, where phase information is important forlocalization. Finally, without using in-phase magnitude-complementarycrossover filters, there can be significant ripples in the powerresponse near the crossover frequency.

The methods described facilitate filling in gaps or notches caused byphase cancellations, within the resolution of the ear's auditory filter,while minimizing any undesirable effects. These methods help reduce theperception of comb filter coloration changes that occur when moving thehead. They may also enhance dialogue intelligibility, especially inacoustically dry environments. The mild spatial blurring helps make thecollapse of the phantom image toward the nearest speaker somewhat lessobvious, and it greatly improves the problem of headphone compatibilityby spreading the center image so it does not seem to be located at afixed point in the center of the head.

Although only a few embodiments of the present invention have beendescribed, it should be understood that the present invention may beembodied in many other specific forms without departing from the spiritor the scope of the present invention. The present examples are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope of the appended claims along with their full scope ofequivalents.

While this invention has been described in terms of a specificembodiment, there are alterations, permutations, and equivalents thatfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing both the process andapparatus of the present invention. It is therefore intended that theinvention be interpreted as including all such alterations,permutations, and equivalents as fall within the true spirit and scopeof the present invention.

1. A method of decorrelating a signal using phase diffusion at highfrequencies, the method comprising: separating a mono input signal intoa high-frequency signal and a low-frequency signal; processing thehigh-frequency signal using a first diffusion means to create ahigh-frequency left channel signal and a second diffusion means tocreate a high-frequency right channel signal, wherein afrequency-dependent delay is created between the high-frequency leftchannel signal and the high-frequency right channel signal; processingthe low-frequency signal to create a delayed low-frequency signal; andcombining the delayed low-frequency signal with the high-frequency leftchannel signal and combining the delayed low-frequency signal with thehigh-frequency right channel signal, thereby producing a stereo responsewith phase diffusion at high frequencies.
 2. A method as recited inclaim 1 further comprising accepting a mono input signal.
 3. A method asrecited in claim 1 wherein separating the mono input signal furthercomprises: using a pair of magnitude-complementary filters.
 4. A methodas recited in claim 1 wherein the first diffusion means comprises afirst allpass filter and the second diffusion means comprises a secondallpass filter.
 5. A method as recited in claim 4 further comprising:applying in one of the first allpass filter or the second allpassfilter, a positive feedback gain and a negative feedforward gain,concurrently applying in the other allpass filter a negative feedbackgain and a positive feedforward gain, thereby creating afrequency-dependent delay between the high-frequency right channelsignal and the high-frequency left channel signal.
 6. A method asrecited in claim 1 wherein separating a mono input signal furthercomprises using a high pass filter and a low pass filter.
 7. A method asrecited in claim 1 wherein the second diffusion means is different fromthe first diffusion means.
 8. A method as recited in claim 1 wherein thedelay of the delayed low-frequency signal is substantially the same asan average of delays of the high-frequency left channel signal and thehigh-frequency right channel signal.
 9. A method as recited in claim 1wherein combining the delayed low-frequency signal with thehigh-frequency left channel signal further comprises creating a leftchannel output signal.
 10. A method as recited in claim 1 whereincombining the delayed low-frequency signal with the high-frequency rightchannel signal further comprises creating a right channel output signal.11. A method as recited in claim 1 wherein the frequency-dependent delaydoes not cause significant temporal smearing of impulsive sounds.
 12. Amethod of decorrelating a signal using phase diffusion at highfrequencies, the method comprising: separating a left input signal intoa left high-frequency signal and a left low-frequency signal andseparating a right input signal into a right high-frequency signal and aright low-frequency signal; applying a first diffusion means to the lefthigh-frequency signal, thereby creating a diffused left high-frequencysignal; applying a second diffusion means to the right high-frequencysignal, thereby creating a diffused right high-frequency signal;creating a delayed left low-frequency signal and a delayed rightlow-frequency signal; combining the delayed left low-frequency signalwith the diffused left high-frequency signal; and combining the delayedright low-frequency signal with the diffused right high-frequencysignal, thereby producing a stereo response with phase diffusion at highfrequencies.
 13. A method as recited in claim 12 further comprisingaccepting a left input signal and a right input signal.
 14. A method asrecited in claim 12 wherein the first diffusion means includes a firstallpass filter and the second diffusion means includes a second allpassfilter.
 15. A method as recited in claim 14 further comprising: applyingin one of the first allpass filter or the second allpass filter, apositive feedback gain and a negative feedforward gain, concurrentlyapplying in the other allpass filter a negative feedback gain and apositive feedforward gain, thereby creating a frequency-dependent delaybetween the diffused left high-frequency signal and the diffused righthigh-frequency signal.
 16. A method as recited in claim 12 wherein thefirst diffusion means is different from the second diffusion means. 17.A method as recited in claim 12 wherein a delay of the delayed leftlow-frequency signal is substantially the same as an average of delaysof the diffused left high-frequency signal and the diffused righthigh-frequency signal;
 18. A method as recited in claim 12 wherein adelay of the delayed right low-frequency signal is substantially thesame as an average of delays of the diffused left high-frequency signaland the diffused right high-frequency signal.
 19. A method as recited inclaim 12 wherein combining the delayed left low-frequency signal withthe diffused left high-frequency signal creates a left channel outputsignal and combining the delayed right low-frequency signal with thediffused right high-frequency signal creates a right channel outputsignal.
 20. A system for decorrelating a mono input signal using phasediffusion at high frequencies, the system comprising: a high pass filterfor outputting a high-frequency signal from the mono input signal; a lowpass filter for outputting a low-frequency signal from the mono inputsignal; a first diffusion means for creating a high-frequency leftchannel signal; a second diffusion means for creating a high-frequencyright channel signal; and a delay component for creating a delayedlow-frequency signal.
 21. A system as recited in claim 20 furthercomprising: a first adder for combining the delayed low-frequency signaland the high-frequency left channel signal.
 22. A system as recited inclaim 20 further comprising: a second adder for combining the delayedlow-frequency signal and the high-frequency right channel signal.
 23. Asystem as recited in claim 20 further comprising: a first gain componentand a second gain component.
 24. A system as recited in claim 20 whereinthe first diffusion means includes a first allpass filter and the seconddiffusion means includes a second allpass filter.
 25. A system asrecited in claim 20 wherein the first diffusion means is different fromthe second diffusion means.
 26. A system as recited in claim 20 whereina frequency-dependent delay is created between the high-frequency leftchannel and the high-frequency right channel and wherein the delay ofthe delay component is substantially the same as an average of delays ofthe first diffusion means and the second diffusion means.
 27. A systemfor decorrelating a stereo input signal having a left input and a rightinput using phase diffusion at high frequencies, the system comprising:a first low pass filter and a first high pass filter, each forprocessing the left input; a second low pass filter and a second highpass filter, each for processing the right input; a first diffusionmeans for creating a high-frequency left channel signal; a seconddiffusion means for creating a high-frequency right channel signal; anda first delay component for creating a delayed low-frequency leftchannel signal and a second delay component for creating a delayedlow-frequency right channel signal.
 28. A system as recited in claim 27further comprising: a first adder for combining the high-frequency leftchannel signal and the delayed low-frequency left channel signal.
 29. Asystem as recited in claim 27 further comprising: a second adder forcombining the high-frequency right channel signal and the delayedlow-frequency right channel signal.
 30. A system as recited in claim 27wherein the first diffusion means includes a first allpass filter andthe second diffusion means includes a second allpass filter.
 31. Asystem as recited in claim 27 wherein the first diffusion means isdifferent from the second diffusion means.